Modern sensors vary greatly in principle and structure. How to rationally select a sensor based on the specific measurement purpose, the object being measured, and the measurement environment is the first problem to be solved when measuring a certain quantity.
Once the sensor is determined, the corresponding measurement methods and equipment can also be determined. The success or failure of the measurement results largely depends on whether the sensor selection is appropriate.
1. Determine the sensor type based on the object being measured and the measurement environment.
To perform a specific measurement, the first thing to consider is what kind of sensor to use, which requires analyzing many factors before a decision can be made.
Because even when measuring the same physical quantity, there are various sensor principles available, determining which principle is more suitable depends on the characteristics of the measured quantity and the sensor's operating conditions, and requires consideration of the following specific issues:
① The size of the measuring range;
② Requirements for sensor size at the measured location;
③Is the measurement method contact or non-contact?
④ Signal extraction method: wired or non-contact measurement;
⑤ The source of the sensor: domestic or imported, affordable price, or whether it should be developed in-house. After considering these questions, the type of sensor to choose can be determined, and then the specific performance specifications of the sensor can be considered.
2. Sensitivity Selection
Generally, within the linear range of a sensor, higher sensitivity is desirable. This is because only with high sensitivity can the output signal corresponding to changes in the measured quantity be relatively large, which is beneficial for signal processing. However, it should be noted that high sensor sensitivity also makes it easier for external noise unrelated to the measured quantity to enter the sensor and be amplified by the amplification system, affecting measurement accuracy. Therefore, the sensor itself should have a high signal-to-noise ratio to minimize interference signals introduced from the outside.
Sensor sensitivity is directional. When the measured quantity is a single vector and its directionality is critical, a sensor with low sensitivity in other directions should be selected. If the measured quantity is a multi-dimensional vector, the lower the cross-sensitivity of the sensor, the better.
3. Frequency response characteristics
The frequency response characteristics of a sensor determine the frequency range of the measurement. It is necessary to maintain distortion-free measurement conditions within the allowable frequency range. In reality, the response of a sensor always has a certain delay, and it is desirable to minimize the delay time.
A sensor with a high frequency response can measure a wide range of signal frequencies. However, due to the influence of structural characteristics, mechanical systems have greater inertia, so sensors with low frequencies can measure lower frequencies of signals.
In dynamic measurements, the response characteristics of the signal (steady-state, transient, random, etc.) should be considered to avoid excessive errors.
4. Linear range
The linear range of a sensor refers to the range within which the output is proportional to the input. Theoretically, within this range, the sensitivity remains constant. The wider the linear range of a sensor, the larger its measurement range and the better it guarantees a certain level of measurement accuracy. When selecting a sensor, once the type of sensor is determined, the first thing to consider is whether its measurement range meets the requirements.
However, in reality, no sensor can guarantee absolute linearity; its linearity is relative. When the required measurement accuracy is relatively low, within a certain range, a sensor with small nonlinear errors can be approximated as linear, which greatly simplifies the measurement process.
5. Stability
The ability of a sensor to maintain its performance unchanged after a period of use is called stability. Besides the sensor's own structure, the main factor affecting the long-term stability of a sensor is its operating environment. Therefore, for a sensor to have good stability, it must have strong environmental adaptability.
Before selecting a sensor, its operating environment should be investigated, and a suitable sensor should be selected based on the specific operating environment, or appropriate measures should be taken to reduce the impact of the environment.
Sensor stability has quantitative indicators. After exceeding its service life, it should be recalibrated before use to determine whether the sensor's performance has changed. In some applications where sensors are required to be used for a long time and cannot be easily replaced or calibrated, the stability requirements for the selected sensors are even more stringent, and they must be able to withstand long-term testing.
6. Accuracy
Accuracy is a crucial performance indicator for sensors, and it's a vital factor affecting the overall measurement accuracy of a measurement system. Higher accuracy sensors are generally more expensive. Therefore, a sensor's accuracy only needs to meet the overall accuracy requirements of the measurement system; it's unnecessary to choose an excessively high-accuracy one. This allows for the selection of a cheaper and simpler sensor from among many options that meet the same measurement purpose.
If the purpose of the measurement is qualitative analysis, a sensor with high repeatability is sufficient, but a sensor with high absolute accuracy is not recommended. If the purpose is quantitative analysis and precise measurement values must be obtained, a sensor with an accuracy class that meets the requirements should be selected.
For certain special applications where a suitable sensor cannot be selected, it is necessary to design and manufacture the sensor in-house. The performance of the self-made sensor should meet the application requirements.
